In keeping with the prevailing standards of the time, Torvald is the undisputed head of the household. He is the sole breadwinner for his family and has complete control over the household finances. The respective roles of husband and wife in this relationship are completely conventional, just what would be expected in a respectable, middle-class marriage. While he goes out to work each day at the bank, his wife, Nora, is expected to stay at home and look after the children.
Ibsen skillfully uses language to heighten this enormous disparity in power between Torvald and Nora. Torvald infantilizes his wife by giving her childish, patronizing nicknames, such as "my little skylark" and "my squirrel." Torvald is only able to get away with using such condescending language because he is the undisputed head of the house.
His position of power gives him the authority to define who Nora is, and Torvald has chosen to use his authority as husband to define Nora as little more than a child. Language plays a very important part in this process. Describing Nora in such infantilized terms allows Torvald to keep her in a state of subjection; his own children may one day grow up, but Torvald is determined that Nora never will. Torvald may think he's protecting Nora by treating her this way, but in actual fact, he's stunting her moral and intellectual growth, with potentially damaging consequences.